fix shadow play: only use ahead extension set per direction#21
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Shadow play was ANDing both left and right extension sets at
every position. The "behind" extension set (based on board tiles
already traversed) can be more restrictive than the GADDAG
allows, because the extension set doesn't account for rack tiles
placed during word generation. This made the shadow play upper
bound too tight, potentially missing valid moves.
Fix: shadow_play_right uses only left_extension_strip (ahead).
shadow_play_left uses only right_extension_strip (ahead).
Word generation already uses one extension set per direction.
Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>
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Summary
Fixes a correctness bug introduced in #15 (extension sets).
Shadow play was ANDing both left and right extension sets at every position. The "behind" extension set can be more restrictive than the GADDAG allows (it doesn't account for rack tiles placed during word generation), making the upper bound too tight and potentially missing valid moves.
Fix: use only the "ahead" extension set per shadow direction — left_extension_strip for shadow_play_right, right_extension_strip for shadow_play_left. Matches what word generation already does.
Test plan